1. Field of the Invention
This invention relates to a hydraulic directional-control valve and, more particularly, a hydraulic directional-control valve of the kind defined in the precharacterising part of the independent claim.
2. Description of the Related Art
In a commonly used prior art embodiment of a directional-control valve of this kind, the valve member is a valve spool which is axially displaceable in a bore in a valve housing.
In the neutral position of the valve spool, an annular circumferential groove in the valve spool forms part of a flow path between one of the inlet ports and a tank port for discharging hydraulic fluid to a reservoir. At the same time, the valve spool blocks flow paths extending from the other inlet port to respective ones of a pair of user or work ports adapted for connection to a driven member, such as to opposite sides of a hydraulic motor, which may be a double-acting hydraulic cylinder, for example.
When the valve spool is displaced to either of two operating positions corresponding to respective operating directions of the hydraulic motor, a second annular circumferential groove in the valve spool forms part of a flow path between the second inlet port and one of the work ports, and a third annular circumferential groove in the valve spool forms part of a flow path between the other work port and the tank port. At the same time, the valve spool blocks the flow path between the first inlet port and the tank port.
In use of the valve, hydraulic fluid is fed to it from a pump which is connected to the two inlet ports by way of a flow divider. A first outlet of the flow divider is provided with a restrictor and connected to both inlet ports to make available at these ports a fluid flow the flow rate of which is determined by the restrictor. A second outlet of the flow divider opens to the reservoir by way of a valve which is controlled by the pump output pressure.
If the valve spool is in the neutral position, the pump feeds a fluid flow of a given flow rate to the first inlet port through the first flow divider outlet. This flow is passed to the reservoir without causing a substantial pressure drop between the first inlet port and the tank port. Because the valve spool blocks the first inlet port, no fluid flow is passed through that port. That portion of the fluid flow from the pump which is not passed through the first inlet port is discharged to the reservoir through the second flow divider outlet.
This embodiment of the prior art directional-control valve functions well as long as the flow rate to be handled by the valve is not substantially different from the flow rate for which the valve is designed. In many applications, however, the actual flow rate may differ substantially from the dimensioning flow rate and in such cases the control characteristics of the valve, in terms of flow-rate change versus displacement of the valve spool from the neutral position, are unsatisfactory.
An object of the invention is to provide a directional-control valve of the kind defined in the precharacterising part of the independent claim which may be optimized for two different operating flow rates and may be switched in a simple manner between two corresponding modes of operation.
In accordance with the invention this object is achieved by the construction of the valve set forth in the characterising part of the independent claim. Features of preferred embodiments of the valve are set forth in the dependent claims.
The invention will be described in greater detail below with reference to the accompanying drawings which show an embodiment by way of example.